An Etymological Dictionary of Astronomy and Astrophysics

The end of a solar eclipse marked by the disk of the Moon completely passing away from the disk of the Sun.

Etymology (EN): From M.E. fourthe, O.E. féowertha, from four, from
O.E. feower, from P.Gmc. *petwor- (cf. Du. and Ger. vier, O.N. fjorir, Dan. fire, Sw. fyra), from PIE *qwetwor (cf. Mod.Pers. cahâr, Av. caθwar-, catur-, Skt. catvarah, Gk. tessares, L. quattuor)

• -th a suffix used in the formation of ordinal numbers, from
  M.E. -the, -te, O.E. -tha, -the; cf. O.N. -thi, -di; L. -tus; Gk -tos; → contact.

Etymology (PE): Parmâs, → contact; cahârom cardinal form from cahâr “four,” cognate with E. four, as above.